ADAM Audio has set a problem for itself by describing its tweeter as a kind of ribbon tweeter. It seems, judging from Internet comments, that many people think it is a ribbon tweeter. But it isn't, quite.
What is normally regarded as a ribbon tweeter has a flat metallic diaphragm suspended in a magnetic field. Its advantage over the more usual dome tweeter is that the ribbon works as both radiating diaphragm and coil, with every part of the ribbon being driven directly. It can have a frequency response up to 100 kHz that only your pet bat can hear.
Adam Audio's tweeter, although it has the word 'ribbon' in its name, is more properly called an 'air motion transformer', as invented several decades ago by Dr. Oskar Heil.
The essential difference between this and a conventional drive unit is that the diaphragm of a conventional driver moves as fast as the air in front is required to move to create the desired acoustic wave. The air motion transformer can however move air four times faster than the speed of the diaphragm.
Adam Audio has incorporated this technology into its X-ART tweeters. The diaphragm is folded into a concertina shape that is squeezed by the incoming audio signal. Compression and expansion of the folds of the concertina force the air in and out much faster than the motion of the diaphragm itself.
The advantages of this are, according to ADAM Audio, that the tweeter has "unprecedented clarity and pristine transient reproduction" and also "avoids the typical breakup/distortion and subsequent dynamic limiting at higher frequencies of stiffer voice coil designs". A further bonus is that the X-ART's equivalent of a coil is in direct contact with the air and thus can be cooled more efficiently.
Also, by folding the diaphragm, it can cover a much larger area while remaining compact in terms of its aperture to the air. A smaller diaphragm aperture has a larger angle of dispersion, which is a desirable feature for a tweeter in many applications.
Whether this technology can be shown to be subjectively better than a conventional tweeter will be down to its ultimate acceptance in the pro-audio marketplace. Or not. It is however a fascinating development and, as always, we applaud technical progress.
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